1. Field of the Invention
The present invention relates to a thermal barrier coating material, a thermal barrier member, and a member coated with thermal barrier which exhibit excellent durability, a method for manufacturing the member coated with thermal barrier, and a gas turbine. More specifically, the present invention relates to the structure of a ceramic layer used as a top coat of the member coated with thermal barrier.
2. Description of the Related Art
In recent years, the elevation of the thermal efficiency of thermal power generation has been studies as one of energy saving measures. In order to improve the power generation efficiency of a gas turbine, the elevation of the gas inlet temperature is effective, and in some cases, the temperature is elevated to about 1500° C. In order to realize the temperature elevation in power generators, it is required in the gas turbine to construct a stationary blade, moving blade or the wall of the burner with heat-resistant members. However, although the material of the turbine blades is a refractory metal, it cannot resist such a high temperature. Therefore, the refractory-metal substrate is protected from high temperatures by forming a thermal barrier coating (TBC). The thermal barrier coating is formed by laminating ceramic layers composed of oxide ceramics on the refractory-metal substrate through a metal bonding layer using a film-forming method such as thermal spray coating. As the ceramic layer, a ZrO2-based material, especially a YSZ (yttria-stabilized zirconia) in which ZrO2 is partially or completely stabilized with especially Y2O3, is often used because it is a ceramic material having a relatively low thermal conductivity and a relatively high coefficient of thermal expansion.
It is considered that the inlet temperature of the gas turbine can be elevated to a temperature higher than 1500° C. in some types of gas turbines. When the gas turbine is operated at such a high temperature and even if the stationary blade and the moving blade of a gas turbine are coated with a thermal barrier coating material comprising the ceramic composed of YSZ, there has been a risk that a part of the ceramic layer is peeled off and heat resistance is lost during the operation of the gas turbine under severe operating conditions. In recent years, it is expected in view of higher thermal efficiency that inlet temperature of the turbine reaches as high as 1700° C. so that the surface temperature of turbine blades elevates to as high as 1300° C. Therefore, further higher heat resistance has been required for the thermal barrier coating of turbine blades.
The above-described problem of the separation of the ceramic layer of YSZ is caused by that the stability of YSZ crystals is insufficient in a high-temperature environment, and the YSZ crystals do not have sufficient durability against a large thermal stress. In order to solve such a problem a member coated with thermal barrier coating material comprising a zirconia layer stabilized with Yb2O3, or a zirconia ZrO2 layer stabilized with Yb2O3 and Er2O3 has been proposed in Japanese Patent Application Unexamined Publication No. 2003-160852.